IIRC this is the reason we are setting `default_schedule_zone` in nova.conf so
that the zone is populated explicitly in the request_spec in the database so that
it can’t be moved to another zone later, but im not 100% sure.

Obviously the drawback being that you populate one zone only if none is
selected.

I don’t think there is an option to “schedule to any zone and then hardcode it
in request_spec”, we would be interested if that’s available today or something
that could be added.